multi_delete: make content-md5 mandatory

The AWS S3 document says that the content-md5 header must be used for the
Multi-Object Delete operation.  This patch add a change to follow the
specification.

Change-Id: I0b289a0302e6465a23acb0acbfa1636b1ef91e5b
cfg: convert types automatically based on default values

The type of the config value is string by default and we need to cast it to the
appropriate type - it is error-prone.

The swift3 middleware actually knows the correct config type from the default
value in swift3.cfg and there is no need to convert it manually.

Change-Id: I28761ed12c3bdd85cb2a80048d2abc0dd23222b3
Merge "multi_delete: make content-md5 mandatory"
Remove unused variable

Change-Id: Ie6b8144346ffc8861affa5c5708f5702b9b7e5c1
Merge "cfg: convert types automatically based on default values"
Merge "Remove unused variable"